The present invention relates to a filtering device for shutting out rays of the ultraviolet and infrared regions, which are harmful to the growth of plants. The filtering device is also used as windows for buildings and vehicles such as automobiles, trains, and ships. In this specification the percentage is represented in terms of weight unless otherwise indicated.
In general, strong sunlight particularly in summer is harmful to the growth of plants. To avoid damage by sunlight, a cover of gauze is used as a covering or alternatively an air conditioning system is employed to control the build-up of heat in greenhouses.
To avoid sunlight entering cabins and rooms of vehicles and buildings, blinds, curtains or the like are commonly used as shades.
However, the problem of the gauze cover is that it shuts out not only infrared rays but also visible rays which are indispensable to the life of plants. The problem of an air conditioning system is that it is difficult to control the temperatures in the greenhouses adequately. In addition, the air conditioning system is costly in maintenance.
In the case of buildings and vehicles the blinds and curtains may be effective to protect the insides against sunlight but it is a drawback to make the rooms and cabins dark and dim.
Against such background the inventor invented a filtering device which shuts out a greater portion of the infrared and ultraviolet rays but allows the passage of visible rays. The device includes a sealed container, which comprises a pair of transparent panels disposed in parallel, the container containing a mixture of water, a compound which is dissociable into ferrous salt ions, and a reducing agent, which is added to prevent the turbidity of the mixture resulting from the ferrous salt ionization into ferric salt ions. Sulfuric acid (H.sub.2 SO.sub.4), uric acid (C.sub.5 H.sub.4) or the like is used as a reducing agent, and it has been found that such substance, whichever may be used, presents problems. More specifically, strong acids such as sulfuric acid cause contamination problems when the used filtering device is discarded, and when a substance hard to solve in water, such as uric acid, is used, the filtering device is difficult to make because of the difficult solubility of the substance.
Accordingly, the present invention is to provide a filtering device which shuts out a greater portion of the ultraviolet and infrared rays but allows the passage of visible rays, thereby enabling the filtering device to be applied to the culturing of plants and the indoor lighting in buildings and vehicles.
In order to achieve this object the inventor has made further research and found that when at least one of L-ascorbic acid and D-ascorbic acid is used instead of the reducing agent it prevents the turbidity or undesired coloring caused by the ferrous salt ionization into ferric salt ion. In addition, so long as these substances are in ordinary use, they are unlikely to cause the problem of contamination and involve no difficulty in manufacturing the filtering devices because of the good solubility in water.